Surveillance techniques and systems for detecting the presence and bearing of objects in space can be categorized, generally, as either passive or active. Typically, active systems require some type of electromagnetic energy emission, such as in the case of radar or laser systems. Passive systems on the other hand, rely on sensors which respond to natural phenomena or signal emissions from objects.
One type of passive space surveillance system senses sunlight reflected off objects or targets. Thus, while this technique may be suitable for daytime detection, it does not provide nighttime detection capability. That is, when a target is located in the earth's shadow, visible light sensing is unsuitable since there is no reflected visible light with which to sense a target.
One passive surveillance system for detecting targets located in the earth's shadow utilizes Long Wave Infrared (LWIR) detectors which rely on the self-emission of infrared radiation by targets. The resolution of LWIR detectors is equal to the product of a diffraction limit and the distance between the detector aperture and the target (i.e., the target range). More particularly, the diffraction limit is inversely proportional to detector aperture size (i.e., 1.22.lambda.aperture size). Thus, the resolution is bounded by feasible aperture size. For example, a LWIR detector operating at a wavelength of ten microns and having a 0.5 meter aperture, has a resolution of approximately forty-five meters for targets located at a range of one-thousand nautical miles. With such an arrangement, the detection of closely spaced objects (i.e., for example, those spaced closer than forty-five meters) may pose a problem. Moreover, LWIR detectors generally require target temperatures of at least three-hundred degrees Kelvin for detection. Thus, detection may be avoided by cooling a target. Furthermore, LWIR detector elements can be complex and costly to manufacture.